1. Field of the Invention
The present invention relates to catalysts for manufacturing carbon substances which are carbon nanocoils having an external diameter of 1000 nm or less, or carbon nanotubes having a sectional diameter of nanosize, etc. More specifically, the present invention relates to the catalysts for manufacturing carbon substances which effectively cause the carbon substances to grow on the surface of the catalysts while a raw-material gas is under pyrolysis by means of a catalyst which contains at least more than two components, that is, which contains at least both a first element group including iron, cobalt and nickel and a second element group including tin and indium.
2. Prior Art
Diamond and graphite have been known as substances made from carbon (hereafter called “carbon substance”). The crystal structure of a diamond is stereo-structure and the crystal structure of the graphite is a two dimensional layer-structure. The utility of these two carbon substances is extremely limited due to the difficulty of technical treatment for them.
In order to utilize the heat resisting property and the strength of carbon, a research and development of carbon filament started actively in 1960. By weaving carbon filaments to form a woven sheet, and further by combining the woven sheet and resin to make a compound fiber, a compound fiber can be utilized in a wide region.
Since the method of manufacturing carbon filaments was established in the decade of 1980, it is succeeded to give car bodies lightness and strength by means of making car bodies from the carbon filaments. Furthermore, golf equipment and fishing rods which are formed with compound fibers have been brought into practical use. Thus many kinds of carbon goods have been used.
There are two methods for manufacturing carbon filaments, one is to remove the organic substance by means of calcinating organic fibers such as acrylic fiber, etc, another is a vapor-phase catalytic decomposition method in which the growth of carbon filaments is stimulated by means of decomposing hydrocarbon in gas phase by using catalytic particles.
Specifically, in the vapor-phase catalytic decomposition method, the fine powder of ferromagnetic metal such as Fe, Ni and Co is used. In this method the carbon filament is grown at the tip-end on which this catalytic powder adheres, while pyrolizing hydrocarbon at the tip end portion. Besides, a method that uses the catalytic powder of Fe.Co alloy is developed. However, most of the carbon filaments which are manufactured in these methods are curved in the middle portion, and it was difficult to grow carbon filaments with high linearity.
In such a situation, a discovery of fullerene was reported in the Nature magazine, Vol. 318 (1985) 162, by H. W. Kroto, J. R. Heath, S. C. O'Brien, R. F. Curl, and R. E. Smalley, which is expressed by C60 and a carbon molecule of a soccer ball shape. This fullerene is a new type of the carbon substances.
Nest, S. Iijima reported in 1991, in the Nature magazine; Vol. 354 (1991) 56-58 that he succeeded in the synthesis of a carbon nanotube with high linearity by means of an arc discharge method. The characteristic point of his method was to find carbon filaments with very high linearity in carbon substances heaped up on the cathode, which were produced by the ordinary arc discharge without using any catalyst. He named helical micronanotube the carbon filament, but at present, it is called a carbon nanotube.
Though the use of fullerene is not so much, the development for use of the carbon nanotube has been rapidly extended. In this circumstance, a carbon nanocoil, in addition, was discovered as a further new carbon substance. The carbon nanocoil was discovered in the process of researching carbon microcoil.
The fact that carbon fibers grow in a vapor phase, while being twisted in the manner of a rope, was first reported by Davis et al. (W. R. Davis, R. J. Slawson and GR. Rigby, Nature, Vol. 171, 756 (1953)). Since the external diameter of such carbon ropes is micro-size, such ropes are ordinarily referred to as carbon microcoils. Subsequently, various reports appeared concerning carbon nanocoils: however, since there was strong element of randomness involved in the production of such coils, the coils lacked reproducibility, and reminded in a state that was inadequate for industrial production.
In 1990, Motojima et al. (S. Motojima, M. Kawaguchi, K. Nozaki, and H. Iwanaga, Appl. Phys. Lett., 56 (1990) 321) discovered an efficient method for manufacturing carbon microcoils, and as a result of subsequent research, they established a manufacturing method that showed reproducibility. In this method, a graphite substrate which is coated with a powdered Ni catalyst is placed inside a horizontal type externally heated reaction tube made of transparent quartz, and a raw-material gas is introduced perpendicularly onto the surface of the substrate from a raw-material gas introduction part located in the upper part of the reaction tube. This raw-material gas is a mixed gas of acetylene, hydrogen, nitrogen and thiophene. The exhausted gas is discharged from the bottom part of the reaction tube.
In this manufacturing method, impurities such as sulfur and phosphorous, etc., are indispensable; and if the amounts of these impurities are too large or too small, carbon microcoils will not grow. For example, the coil yield reaches maximum, at a value of approximately 50%, in a case where thiophene-containing sulfur is added at the rate of 0.24% relative to the total gas flow. The reaction temperature is approximately 750 to 800° C.
Diameter of fibers constituting such carbon microcoils is 0.01 to 1 μm, the external diameter (outside diameter) of the coil is 1 to 10 μm, the coil pitch is 0.01 to 1 μm, and the coil length is 0.1 to 25 mm. It is characteristic that these carbon microcoils are of micro-size and have an amorphous structure. In another ward, the carbon microcoils are substances that amorphous fibers grow up in a coil shape without a hole.
In 1991, carbon nanocoils were discovered. Spurred by this discovery, research concerning carbon coils on the nanometer scale, i.e., carbon nanocoils was initiated. The reason for this was that on the nanometer scale, there was a possibility that a new physical property might be discovered, so that such nanocoils showed promise as new materials in electronics and engineering, etc., in nanometer region.
In 1994, Amelinckx et al. (Amelinckx, X. B. Zhang, D. Bernerts, X. F. Zhang, V. Ivanov and J. B. Nagy, Science, 265 (1994) 653) succeeded in producing carbon nanocoils. It was also demonstrated that while carbon microcoils are amorphous, carbon nanocoils have a graphite structure. Various types of carbon nanocoils were manufactured, and the minimum external diameter of these nanocoils was extremely small, i.e., approximately 12 nm.
The manufacturing method used by the above mentioned researchers was a method in which a metal catalyst such as Co, Fe or Ni is formed into a fine powder, the area around this catalyst is heated to a temperature of 600 to 700° C., and an organic gas such as acetylene or benzene is caused to flow through so that this gas contacts the catalyst, thus breaking down these organic molecules. The substance produced as a result consists of carbon nanotubes with a graphite structure, and the shapes of these nanotubes are linear, curvilinear, planar spiral and coil form, etc.
In 1999, Li et al. (W. Li, S. Xie, W. Liu, R. Zhao, Y. Zhang, W. Zhou, and G. Wang; J. Material Sci., 34 (1999) 2745) succeeded again in producing carbon nanocoils. In the manufacturing method used by these researchers, a catalyst formed by covering the outer circumference of a graphite sheet with iron particles was placed in the center, and the area around this catalyst was heated to 700° C. by means of a nichrome wire. This catalyst is a 2-component type catalyst consisting of graphite and iron. However, as the carbon nanocoils are carbon substance, the graphite is used as a basic material, so that this catalyst can be regarded as a 1-component Fe catalyst combined with graphite. This manufacturing method also showed a small coil production rate and was extremely inadequate as an industrial production method.
As described above, the catalysts for manufacturing carbon filaments and carbon nanocoil are limited to a one component type catalyst for which ferromagnetic metal such as Fe, Co or Ni is used as simple substance, to a two component type catalyst which is an alloy consisting of two ferromagnetic metals such as Fe.Co, or to a two component catalyst combining ferromagnetic metal and graphite.
In the course of analyzing such a conventional catalyst, the present inventors investigated a possibility of two and three component type catalyst and multi (more than three) component type catalysts which are constructed by adding non-ferromagnetic metals to the ferromagnetic metal such as Fe, Co, or Ni. As a result, the inventors reached to the invention which was published in Japanese Patent Application Laid-Open (Kokai) No. 2002-192204. This invention disclosed a 3-component type catalyst comprising indium, tin and iron.
In more concrete terms, the 3-component type catalyst is formed by vacuum-evaporating an iron thin film on the surface of an ITO substrate which is a thin film being mixture of indium-oxide and tin-oxide. ITO is an abbreviation of Indium-Tin-Oxide. The ITO substrate is widely used as a raw material of semi-transparent electrodes in the field of semiconductor. It was found in a study that, when hydrocarbon gas is pyrolyzed in a reaction apparatus, a large amount of carbon nanocoils grow on the surface of the 3-component type catalyst.
The study shows that there is a possibility of synthesis of a large amount of carbon substances such as carbon nanotubes or carbon nanocoils by using adequate catalysts. There, however, might still exist unknown catalysts. Therefore, it would be very significant to consider again the construction of catalysts.